The present invention relates to a circuit for driving an electric actuator, in particular an electric fuel injector for an internal-combustion engine.
Such a control circuit is schematically shown in FIG. 1 of the accompanying drawings.
In this figure, 1 denotes a direct voltage source, for example a battery in a motor vehicle. In the embodiment shown the voltage source 1 has its negative pole connected to earth GND and its positive pole connected to a chopper circuit denoted overall by 2. An electric actuator 3, such as an electric fuel injector of an internal-combustion engine, is connected to the chopper circuit 2.
4 denotes an electronic control unit designed to apply to the chopper circuit 2, for an activation time ti which is in each case predetermined and included between an initial instant ts and a final instant te, a command signal CS essentially of the PWM (Pulse-Width Modulation) type, with a predefined ON/OFF switching period p and a variable duty cycle. Correspondingly, the chopper circuit 2 causes the flow, within the electric actuator 3, of a correspondingly switched current I having a nominal duration corresponding to the abovementioned activation time ti.
In control circuits for driving electric injectors, the current switching frequency is not particularly high so that the current has a ripple of notable amplitude.
If the duration of each fuel injection operation is controlled in an open loop, the time required for the injector to close completely depends on the instantaneous value reached by the current at the end of the electric injector activation time. In FIG. 2 of the accompanying drawings, the upper graph shows the qualitative progression of the current I in an electric injector driven with a circuit according to the prior art and the lower graph shows the progression of the command signal CS correspondingly applied to the chopper circuit associated with the electric injector. As can be seen, in the example of progression illustrated the current I has a rise phase of duration tr followed by a first switching phase of the peak value, of duration tp, in turn followed by a subsequent switching phase in the region of a maintenance value, of duration th. The signal CS is high (ON) during the phase tr and then has a progression of the ON/OFF type during the phases tp and th, with a constant switching period p and with a high duty cycle during the phase tp and low duty cycle during the phase th.
The command signal CS, started at the initial instant ts, terminates at the final instant te. At the instant te in general the intensity of the current I may be between a minimum value IHMIN and a maximum value IHMAX. Correspondingly, complete closing again of the electric injector occurs at a instant variable within a time interval E which represents the error in determination of the actual injection time.